Magnetorheological fluids are liquid materials formed by magnetic particles in stable suspension in a support liquid. These materials have a very low electrical conductivity when the support liquid is an ionic liquid, and they may be insulating when the support liquid is an organic solvent.
Various attempts have been made to confer a conductive character on magnetorheological materials for the purpose of broadening their field of application. For example, F. E. Luborsky (J. of the Electrochem. Soc., Vol. 108, No. 12, 1961, pp. 1138-1145) describes the introduction of Fe into mercury in an electrochemical cell, the cathode of which is a mercury film and the electrolyte is a solution of an iron salt. The intended aim is to produce a permanent magnet.
S. W. Charles, et al. [Thermomechanics of magnetic fluids (1975), Hemisph. Publ. Corp. Washington 1978, pp. 27-43] describes the preparation of a ferrofluid by a method consisting in introducing Fe electrochemically into Hg or into an Hg/Sn amalgam, using an electrolyte containing an Fe salt. The Fe particles formed on the surface of the cathode are subjected to stirring in order to promote their dispersion in the Hg or in the amalgam. It is observed that the addition of Sn to Hg significantly improves the stability of the ferrofluid system but a certain degree of agglomeration of the Fe particles persists.
Suspensions of nickel particles in a conductive liquid have been described by I. Ya. Kagan, et al (Magnitnaya Gidrodinamika, Vol. 6, No. 3, pp. 155-157, 1970). These suspensions were prepared by introducing nickel particles having a size of about 50 μm into a metallic liquid, namely tin, which is liquid at a temperature above 232° C., bismuth, which is liquid at a temperature above 271° C., or an In—Ga—Sn alloy denoted by Ingas, which is liquid at a temperature above 11° C. or 15.8° C. depending on the composition. According to the authors, the ferrofluid suspensions having these compositions could be obtained by simple mixing of the constituents, because there is a certain wettability of the nickel by the metallic liquids in question and because of the similar densities of nickel and said metallic liquids. However, such a method is not applicable to the production of a conductive ferrofluid in which the metal constituting the magnetic particles and the metal constituting the conductive liquid exhibit little or no mutual affinity and the wettability of the metal constituting the magnetic particles by the conductive liquid metal is low, or zero.
S. Linderoth, et al. (J. Appl. Phys. 68(8), 15/04/1991, pp. 5124-5126) describe two methods of preparing mercury-based ferrofluids. According to the first method, mixed (Fe—Co—B, Fe—Ni—B, Fe—B, Co—B or Ni—B) particles are prepared by adding an aqueous NaBH4 solution drop by drop to an aqueous solution containing ions of the transition metals in question, mercury is then added to the aqueous suspension of mixed particles obtained, and the mixture is subjected to stirring. This first method makes it possible to obtain a suspension of the aforementioned mixed particles in Hg (on condition that the particles are not washed with distilled water before they are introduced into the Hg when they are Fe—Ni—B, Co—B or Ni—B particles). However, the NaBH4 compound that is added is incorporated, by its nature, into the chemical composition of the final product, and the method cannot therefore be generalized to other “magnetic compound/conductive liquid” pairs. According to the second method, metallic iron is dissolved in concentrated HCl, HgCl2 is added to the solution, the pH is adjusted to about 3 by the addition of an appropriate amount of a concentrated aqueous NaOH solution, and then NaBH4 is added to reduce the assembly. In this method, the iron and mercury are generated simultaneously by a chemical process the evolution of which is not controlled. In the general case, it is not certain that there will always be particles, and alloys of uncontrolled composition may form. The method involves having ionic solutions of the metals in question, something which is obviously not always possible. Finally, NaBH4 is a good reducing agent but is not necessarily suitable for all metals.
It is known to use fluids having magnetorheological properties in viscoelastic transmission systems such as, for example, shock absorbers, especially in motor vehicles, antiseismic devices, antivibration devices, bridge decks and clutches. The fluids generally used are formed by magnetic particles of micron size dispersed in liquids, such as synthetic oils or hydrocarbons of low volatility, silicone oils, or aqueous fluids for low-elongation applications with complete sealing. However, these fluids cannot be used in shock absorbers of devices that are subjected to high temperatures, especially above 200° C.